An X-ray phase imaging method is an imaging method which utilizes the phase change induced by a sample as X-rays traverse the sample. One of the several X-ray phase imaging methods proposed in the related art is Talbot interferometry described in, for example, Patent Literature reference 1 (PTL 1).
A Talbot interferometer generally includes two or three gratings each having a periodic structure. Among the gratings, a grating which is generally placed near the sample may be referred to as a “beam splitter grating”, a grating which is generally placed near an X-ray detector may be referred to as an “analyzer grating”, and a grating which is generally placed near an X-ray source may be referred to as a “source grating”. Each of the gratings described above may be a grating having a one-dimensional periodic pattern or a grating having a two-dimensional pattern. The X-ray detector used is generally a detector capable of measuring a two-dimensional intensity distribution of X-rays incident on a detection surface of the X-ray detector.
The beam splitter grating is typically a phase-modulation transmission-based diffraction grating. X-rays incident on the beam splitter grating are diffracted by the periodic structure of the grating, forming an interference pattern (also referred to as a “self-image of the grating”) at a predetermined position due to the so-called Talbot effect. The interference pattern reflects, for example, changes in the phase of propagating X-rays as they traverse the sample and deforms. By performing measurement and analysis of the intensity distribution of the interference pattern, information on the shape and internal structure of the sample may be obtained. In the present invention and throughout the specification, any method for acquiring information on the sample by utilizing X-ray phase changes induced by the sample is referred to as an X-ray phase contrast imaging method even if the information is not converted into an image.
The analyzer grating is typically a grating in which X-ray transmitting portions and X-ray shielding portions are periodically arrayed and thereby having a periodic transmittance distribution. The analyzer grating is placed at the position where the interference pattern described above occurs, and is thus used in order to cause a moiré pattern to appear in the intensity distribution of the X-rays that have passed through the grating. The moiré pattern reflects the deformation of the interference pattern, and the period of the moiré pattern can be increased without limitation. Thus, even if the spatial resolution of the detector used is not high enough to ensure that the interference pattern can be directly detected, detection of a moiré pattern with a large period will enable indirect obtaining of the information on the interference pattern. A Talbot interferometer that utilizes the occurrence of a moiré pattern between the interference pattern and the grating is described in, for example, PTL 1.
As described above, the analyzer grating is used to compensate for insufficient spatial resolution of the X-ray detector. Thus, when a detector having a sufficiently high spatial resolution is used, the use of the analyzer grating is not essential. Since the interference pattern generally has a period of approximately several micrometers (μm), and is too fine to be directly detected with a typical X-ray detector, it is common to use the analyzer grating.
Similarly to the typical analyzer grating, the source grating is also a grating having a structure in which X-ray transmitting portions and X-ray shielding portions are periodically arrayed. The source grating is generally placed near an X-ray emission spot inside the X-ray source (X-ray generator), and is thus used in order to form an array of virtual linear light emitting portions (in a two-dimensional grating, a minute light emitting spots). X-rays emitted from the individual linear light emitting portions formed in the manner described above form a plurality of interference patterns, each described above, and the interference patterns are superimposed on one another while displaced by an integer, said integer being a multiple of the pattern period when no sample or the like is placed in X-ray paths. Accordingly, a periodic pattern having a high X-ray intensity and high fringe visibility can be formed. To achieve such superposition of interference patterns as described above, it is desirable to design each grating so that its grating period and a distance between gratings satisfy predetermined conditions. A Talbot interferometer that uses such a source grating as described above may be particularly referred to as a “Talbot-Lau interferometer”. A Talbot interferometer that uses such a source grating is described in, for example, Patent Literature reference 2 (PTL 2). Hereinafter, the term “Talbot interferometer” is used to also include a Talbot-Lau interferometer.
Using the source grating makes it possible to use an X-ray source having a comparatively large light emitting spot size. If the light emitting spot size is small enough to directly form a high-visibility interference pattern, the use of the source grating is not essential. However, the formation of such a minute light emitting spot in an X-ray tube, which is the most common X-ray source, results in a tendency for the X-ray output per unit time to decrease and the imaging time to significantly increase. Thus, a source grating is generally used when an X-ray tube is used as the X-ray source.
In a Talbot interferometer, it is common to acquire an X-ray transmittance distribution of the sample that is an image based on a principle similar to that of standard X-ray imaging (absorption contrast imaging), and also acquire information on a fringe phase distribution of the interference pattern and a visibility distribution of the interference pattern. In general, the fringe phase and visibility of the interference pattern respectively mainly reflect the spatial differentiation of the phase distribution of the X-rays that have propagated through the sample and the degree of X-ray small-angle scattering caused by fine particles, a fibrous structure, edge portions of an object, or the like.